Research ArticleEffects of Cyclic Loading on the Mechanical Properties of MatureBedding Shale

Yintong Guo , Chunhe Yang, Lei Wang, and Feng Xu

State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy ofSciences, Wuhan 430071, China

Correspondence should be addressed to Yintong Guo; ytguo@whrsm.ac.cn

Received 3 November 2017; Revised 17 January 2018; Accepted 28 January 2018; Published 20 March 2018

Academic Editor: Xiang Fan

Copyright © 2018 Yintong Guo et al. *is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We investigated the mechanical properties of mature bedding shale under cyclic loading conditions, with an application to thedesign of hydraulic fracturing in shale gas wells. Laboratory experiments were conducted on shale samples under two principalloading orientations. Testing results showed that accumulated fatigue damage occurs in a three-stage process. Analysis of fatiguedamage at different maximum stress levels shows that fatigue life increases as a power-law function with maximum stressdecreasing. And the maximum stress significantly affects the fatigue life. Further, the elastic part of shale rock deformation wasrecovered in the unloading process, whereas the irreversible deformation remained. *e irreversible deformation, growth trend,and accumulation of the total fatigue were directly related to the fatigue damage. *is process can be divided into 3 stages: aninitial damage stage, a constant velocity damage stage, and an accelerated damage stage, which accounted for about one-third ofthe fatigue damage. Shale rock is a nonhomogeneous material, and the bedding is well developed. Its fatigue life differs greatly intwo principal loading orientations, even under the same loading conditions. All of these drawn conclusions are of great im-portance for design of hydraulic fracturing in shale gas wells.

1. Introduction

In recent years, with the development of unconventional oiland gas resources, hydraulic fracturing technology has beenpaid more attention. To ensure the success of hydraulicfracturing, it is necessary to understand the mechanicalproperties of shale formations. In such processes, a networkof cracks is obtained through both artificial fracture andnatural cracks in the formation. Under different sedimentaryenvironment conditions, the bedding density of shale for-mation has a great difference. *erefore, the bedding playsa significant role in both mechanical behavior and engi-neering activities [1]. During long horizontal-section drillingand hydraulic fracturing construction engineering, the sur-rounding rock is subjected to loading and unloading states.*emechanical properties of shale under dynamic loads differdramatically from those under static loads. However, thenature of dynamic failure in the rock remains unclear,especially under cyclic loading conditions. *erefore, it is

of great significance to study the mechanical behavior ofshale rock under cyclic loading.

*e long-term stability of rock mass depends on rockfatigue failure mechanisms. Although the deformationcharacteristics and failure mechanism under cyclic loadingare related to the static load, there is a significant differencebetween the two cases.

During the last few decades, a lot of research has focusedon the response of rocks under cyclic loading. *e fatigueproperties of rock materials have been found to be de-pendent on variables such as the amplitude, the upper limitstress, and frequency [2–4]. It has also been reportedthat different materials show different responses whenthey are subjected to cyclic loading—some materials becomestronger and more ductile, while others become weaker andmore brittle [2].

Previous studies [5, 6] show that the fatigue damage ofrock has a “threshold value,” when the maximum stressvalue is lower than a certain value, the sample cannot be

HindawiAdvances in Civil EngineeringVolume 2018, Article ID 8985973, 9 pageshttps://doi.org/10.1155/2018/8985973

destroyed no matter how many load cycles are applied;thus, the maximum stress has an important influence onthe fatigue life. *e mechanical properties of rock samplesunder monotonic and cyclic loadings were also studied [7],it was found that under monotonic loading, thenardite andglauberite samples show an elastic-plastic style; however,under cyclic loading, it changed to a ductile style. *erehave also been several studies on the fatigue properties ofrock salt subjected to interval cyclic pressures [2, 8, 9]. Inone of these studies, an empirical model fitting the re-lationship between fatigue life and interval cyclic pressurewas established. In another, a series of laboratory tests wereperformed to assess the effects of cyclic loading on com-pressive strength, elasticity, and time-dependency of MahaSarakham rock salt, the results indicate that the saltcompressive strength decreased with increasing number ofloading cycles and was best represented using a power-lawequation [8].

From the above study, it is evident that fatigue testshave been carried out for the most common types of rocksin geotechnical engineering. However, the main rocksinvolved are sandstone, marble, granite, salt rock, and soon. *e rock types are fairly homogeneous, and almost nostratified sedimentary rocks are involved. *ere is a greatdifference on deformation mechanism between the ho-mogeneous rock and the layered rock. *e angle betweenthe loading direction and the bedding has an importantinfluence on the mechanical properties and failure modesfor layered rock.

During hydraulic fracturing processes in shale gas res-ervoirs, the changing of drilling mud density causes stressfluctuations in the surrounding rock during long horizontalwell drilling. In the hydraulic fracturing stage, pressurefluctuations in hydraulic fracturing fluid can be clearlyobserved, with more than twenty circulates commonlyobtained. *e shale rock mass is subjected to repeatedloading and unloading cycles; therefore, accurate assessmentof the shale reservoir crushing effect and long-term stabilityof the well-bore during hydraulic fracturing is an importantpractical significance.

*ere are many sets of small layer shale formation withdifferent mechanical properties and bedding densities in JiaoShiba, China. Understanding the fracture characteristics ofshale rock under cyclic loading is of great practical signif-icance to accurately evaluate the fracturing effects andlong-term stability of shale reservoirs. *e objective of thisresearch is to experimentally determine the effects of cyclicloading on mature bedding shale under two principalloading orientations. Using RMT-150C rock mechanics testsystem, the axial deformation, strength characteristics, andrupture mode of the cycle loading were studied and ana-lyzed, and some useful conclusions were obtained whichcould provide effective technical parameters for hydraulicfracturing.

2. Experimental Approach

2.1. Specimens. *e shale samples used in the experimentwere collected from Longmaxi formation at Nanchuanoutcrop in Chongqing Province, China. In this paper, theshale samples adopted with well-developed bedding; in onemeter, the number of bedding is more than 200. *e maincomponents of specimens were quartz, clay minerals, andfeldspar, with a small amount of muscovite. *e mineralcompositions are shown in Table 1.

*e fatigue properties of shale rock were studied undercyclic loading with two principal loading orientations:parallel and perpendicular to the bedding planes, re-spectively. Shale samples with similar P-wave velocitieswere selected for cyclic loading tests. ISRM standards [10]were followed when cutting the samples into standardcylinders with a diameter-to-length ratio of 1 : 2, specimen’sdiameter is 50mm, and its height is 100mm. *e degree ofparallelism between the upper and lower ends is within0.03mm; typical shale samples before testing are shown inFigure 1.*ree shale samples for each coring direction werecarried out on uniaxial compression tests to ensure that theresults were representative, and the residual samples wereused for cyclic loading tests. *e average density was2.665 g/cm3, and the average porosity was 1.25% by themercury intrusion method. *e average P-wave velocity ofvertical coring samples (numbered H) was 4667m/s, andthe average P-wave velocity of parallel coring samples(numbered V) was 4306m/s. Horizontal permeabilitymainly ranged between 0.0338 and 0.0581mD, with anaverage value of 0.04595mD, and vertical permeability waslower than horizontal permeability, with a difference ofmore than 2 orders of magnitude. *e shale samples alsoshow obvious anisotropic characteristics.

Table 1: *e mineral compositions of shale rocks.

Montmorillonite Illite Quartz Cristobalite Albite Calcite Muscovite Pyrite Annite AnkeriteShale 1.66 3.60 50.23 4.75 17.60 2.60 5.71 5.80 3.83 4.21

Figure 1: Typical shale samples before testing.

2 Advances in Civil Engineering

2.2. Experimental Design. Prior to carrying out cyclicloading tests, the mechanical properties of shale samplesunder uniaxial compression tests were measured, as shownin Table 2.

*e mechanical properties of shale are obtained byuniaxial compression test, which provides a reference for theupper limit stress setting of cyclic loading test. Uniaxialcompression tests were performed on three samples eachof vertical and parallel coring, respectively. All the testswere performed using a RMT-150C electrohydraulicservo-control testing machine, as shown in Figure 2.Displacement control mode with a loading rate of0.003mm/s was used for uniaxial compression tests, andloading control mode was used for cyclic loading tests. Forcyclic loading tests, the axial loading was specified asa sinusoidal cyclic compressive load, and the loading fre-quency was set to 0.5 Hz. *e cyclic tests were conductedat four different maximum stress amplitudes on theparallel and vertical coring samples. *e maximum stresslevel (the ratio of maximum cyclic stress to the averageof uniaxial compression strength) was varied from 0.95to 0.80. Initially, the sample was loaded up to the mean-stress level in the displacement control mode, and then thecyclic test was performed at a given amplitude and fre-quency. It should be noted that the predetermined stressamplitude for the machine was equal to one-half of the totalrange [11].

Finally, the results of the fatigue tests were analyzed inrelation to various parameters such as axial strain, cycletimes, and rupture mode. *e fatigue test characteristics areshown in Table 3.

3. Test Results and Analysis

3.1. Uniaxial Compressive Strength andDeformation. Table 2shows the results under uniaxial compression tests. *estress-strain curves are shown in Figures 3 and 4. *e stressshows a maximum value with increasing axial strain beforedecreasing rapidly. *e samples show obvious brittlenesscharacteristics. For two principal loading orientations, thecompressive strength and elastic modulus were obviouslyanisotropic, as there were great differences. For parallelcoring samples, the average peak stress is 86.20MPa, andaverage elastic modulus is 15.95GPa. In these samples, splittensile failure occurred, with the specimen being split intomultiple independent pieces almost parallel to the beddingsurfaces. For vertical coring samples, the average peak stressis 116.49MPa, and average elastic modulus is 15.26GPa. *einitiation of microcracking occurred along the beddingplane, and it gradually runs through the bedding plane andcauses the specimen destruction, resulting in an approxi-mately parallel layered block. Also, the reproducibility ofthese experiments was well, thus providing a basis for se-lection and comparison of the control indexes of the cycletest.

3.2. Comparison between Cyclic Loading and UniaxialCompression. Under cyclic loading, the failure process ofshale rock is different from that under static uniaxialcompression; however, there is some relationship betweenthe two cases [12]. *e axial stress-strain curves underuniaxial compression and cyclic loading are drawn on thesame diagram (for parallel coring samples) in Figure 5. *eupper limit stress ratio of the cycle test was 0.90, and theamplitude stress ratio was 0.65; it shows that the end de-formation is consistent with uniaxial loading failure de-formation. *e figure indicates that the failure occurs belowthe maximum strength loading condition, as a result ofaccumulative damage. *ere are two main stages of axialstrain: stable crack propagation and unstable crack propa-gation, resulting in a sudden breakdown. Figure 6 shows thestress-strain curves for uniaxial compression (sample V-13)and cyclic loading (sample V-3) tests. *e upper limit stressratio of the cycle test was 0.95, and the amplitude stressratio was 0.81. *e end of the cyclic loading deformationwas larger than uniaxial compression failure. In addition,the cumulative deformation in vertical bedding planeswas relatively large. It is analyzed that, under uniaxial

Table 2: Mechanical properties of shale samples under uniaxial compression test.

Sample number Axial stress (MPa) Elastic modulus (GPa) Poisson ratioH-11 87.27 14.66 0.321H-12 84.25 16.69 0.337H-13 87.09 16.49 0.290Average 86.20 15.95 0.32V-11 119.25 15.10 0.282V-12 112.81 15.41 0.281V-13 117.42 15.27 0.262Average 116.49 15.26 0.275

Figure 2: Apparatus and schematic.

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compression, the sample is always in the stage of increasingload, and partial failure of specimen causes the failure of thewhole specimen. However, under cyclic loading andunloading conditions, the local stress state of the beddingplane can be adjusted, so the specimen is fully deformedduring fatigue failure. �e cyclic loading results showed thatthe fatigue failure consisted of three stages: an initial stage of

fatigue fracture (initiation phase I), a stable crack propagation(uniform velocity phase II), and an unstable crack propa-gation, resulting in a sudden breakdown (accelerated phaseIII). �ese three steps have also been reported in previousstudies [13, 14].�e fatigue process is related to accumulativedamage, which oftentimes leads to a progressive weakness ofmaterials. Variation of these parameters with time or cyclenumber can be observed in all three stages of fatigue damage.

3.3.�e E�ect of Maximum Stress. �e relationship betweenmaximum cyclic loading ratio and fatigue damage cyclenumber for two principal loading orientations is illustratedin Figure 7. �e e�ect of maximum stress on fatigue lifeindicates that the relationship between the maximum stressand fatigue life is subordinate to the power-law function. Forparallel coring samples, when the applied maximum stresslevel was 0.95, the sample damaged after 11 cycles. However,when the maximum stress level was 0.80, the failure stressobtained was from 81.75MPa to 68.92MPa, as the rockfatigue damage after 5689 cycles was 517 times that forspecimen H-1. It is evident that the damage grew very slowlyas the maximum stress ratio 0.80 was close to the fatiguethreshold. For vertical coring samples, the failure stressobtained from 116.49MPa to 93.2MPa, with fatigue damageafter 8956 cycles. With the same upper limit for the stressratio, the fatigue life cycle for vertical coring was longer thanthat of parallel coring. Results were shown that the damagefrom cyclic loading on shale rock was weakened when thenormal stress was perpendicular to the bedding plane.

�e relationship between axial strain and relative cycle(ratio between cycle number and failure cycle number) atfour di�erent maximum stress levels 0.95, 0.90, 0.85, and0.80 is illustrated in Figure 8. Figures 8(a) and 8(b) indicatethat, at 0.95 maximum stress levels, during the fatigue life,the axial strain has no obvious turning point. However, forother upper limit stress conditions, the axial strain can bedivided into 3 stages: an initial deformation stage, a constantvelocity deformation stage, and an accelerated deformationstage, with the second stage being dominant.

�e �rst stage, that is, the development of axial strainrate, is fast, as the sample accumulates a large deformation ina short time period; this leads to the second stage, when thestrain rate is relatively stable and the axial strain develop-ment is very slow, resulting in a very long process, whichoccupies most of the fatigue life. In the third stage, the axial

Table 3: Summary of the fatigue test characteristics.

Samplenumber

�e upper limitstress

�e lower limitstress

Maximum stresslevel

Minimum stresslevel

Amplitudelevel

Frequency(Hz)

Number of failurecycles

H-1 81.75 16.4 0.95 0.19 0.76 0.5 11H-5 77.58 21.75 0.9 0.25 0.65 0.5 155H-6 73.27 28.1 0.85 0.33 0.52 0.5 1856H-8 68.92 34.13 0.8 0.40 0.40 0.5 5689V-1 110.64 16.2 0.95 0.14 0.81 0.5 27V-3 104.85 29.12 0.9 0.25 0.65 0.5 452V-4 99.02 38.44 0.85 0.33 0.52 0.5 2569V-10 93.2 47.76 0.8 0.40 0.40 0.5 8956

0

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Figure 3: Stress-strain curves of shale under uniaxial compressiontest (vertical coring).

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Figure 4: Stress-strain curves of shale under uniaxial compressiontest (parallel coring).

4 Advances in Civil Engineering

strain rate increases rapidly, and the specimen is destroyedquickly. And this process takes a very short time, and thenumber of cycles are also few, as the accumulative e�ects ofthe three deformation stages lead to �nal failure. �e curvein the initial deformation stage shows a larger initial axialstrain, while the process of loading and unloading is ac-companied by the slow accumulation of damage. With theincreasing in axial loading, the internal pores and �ssures areclosed, the structure becomes denser, and the overall sti�-ness is improved.

Under cyclic loading, the elastic deformation will berecovered in the process of unloading, but the irreversibledeformation will be remained. �e irreversible deformation,growth trend, and accumulation of total fatigue are directlyrelated to the fatigue damage. Hence, the study of irre-versible deformation of fatigue failure is more scienti�c andprecise.

Figure 9 shows the relationship between irreversible axialdeformation and relative cycle (ratio between cycle numberand failure cycle number). It can be seen that the fatigueprocess of irreversible deformation may be divided into 3

stages: an initial stage, when the development speed is fast;a second stage, after a certain period of time, the develop-ment speed gradually stabilizes, becoming constant; a thirdnear-destruction stage, when the development speed grad-ually accelerates towards destruction. �e irreversible de-formation gradually accumulates with the continuous actionof cyclic loading, until the specimen is destroyed. �erefore,the fatigue failure of shale rock is progressive accumulationof irreversible deformation in axial direction. For parallelcoring samples, the irrecoverable deformation was relativelysmall at the initial stage of the cycle, and irreversible de-formation was mainly produced in the third stage. Forvertical coring samples, the irreversible deformation ratiowas greater than that for parallel coring in the initial stage,and cumulative irreversible deformation was mainly com-pleted in the �rst and second stages.

3.4. Fatigue Damage Evolution Characteristics. Under theaction of the loading, the localized microcracks in thesamples a�ected the strength of rock material before mac-rocracking had occurred. From the point of view of damagemechanics, Lemaitre, considering the failure process, pro-posed the concept of continuum damage mechanics [15].

Using the de�nition of damage variation [16], the dataobtained in this study were used to calculate the relationshipbetween damage variation and cyclic numbers, as shown inFigure 10. It indicates that according to the cyclic fatigue, thedamage may be divided into 3 stages. In the initial stage, thedamage variable increased with increasing the number ofcycles, and this stage accounted for one-third of the totaldamage.�e second stage occurred after a certain number ofcycles, wherein the microcracks were closed as the newgrowth gradually stabilized, and the damage variable in-creased slowly. Once the damage accumulated to a certainextent, a lot of new cracks began to appear and graduallyconnected causing the damage variable to increase rapidly;that was the third stage. Fatigue failure occurred whena large number of cracks coalesced into a macrocrack. Out ofthese stages, the constant velocity occupied most of the

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Figure 5: Stress-strain curves for uniaxial compression (sample H-12)and cyclic (sample H-2) loading tests.

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0.750 2000

y = 1.0187x–0.026

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Figure 7: �e relationship between maximum cyclic loading ratioand fatigue damage cycle number (frequency 0.5Hz).

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fatigue life, although the cumulative amount of deformationwas only one-third of the total. �e remaining two stagesexperienced a relatively short time, but the accumulation ofdeformation during these stages was very large.

At the same time, di�erent coring angle specimensshow di�erent characteristics in Figures 10(a) and 10(b).For parallel coring samples, the �rst stage of the damagevariable is about 0.3, and the slope of second stage is larger.�e trend is similar under di�erent maximum stressconditions. For vertical coring samples, when the appliedmaximum stress level was 0.95, there was no obvious thirdstage of the damage variable. It indicates that, at 0.95loading stress levels, the damage variable and the numberof cycles were directly proportional, and no obviousturning point was observed; therefore, damage was pro-duced continuously with an increase in the number ofcycles. Under other stress levels, the �rst stage of thedamage variable is about 0.4, and the slope of second stageis smaller than parallel coring samples. �ese di�erencesindicate that the initial damage is greater under conditionof the loading direction perpendicular to the beddingsurface.

�e behavior of damage evolution is greatly dependenton the maximum stress, amplitude, and fatigue initialdamage. �e increase of these variables causes the secondstage to be shortened, which is related to the whole fatigueprocess [17].

3.5. Deformation Modulus and the Number of Cycles. Figure11 shows the relationship between the deformation modulusand relative cycle times. It can be seen that, under initialcyclic loading, the deformation modulus increased slightlywith increasing cycle number. �e relationship betweendeformation modulus and relative cycle times may be di-vided into 3 stages. In the �rst stage, the deformationmodulus increased gradually with increasing cycle times, thechange was mainly due to the closure of rock pores andmicrocracks. For vertical bedding coring samples, in initialcycle stage, the increase of deformation modulus is more

than 6GPa. �e main reason is that the bedding plane iscompacted. For parallel bedding coring samples, the increaseof deformation modulus is smaller. It is mainly becausethe loading direction is parallel to the bedding plane, andthe e�ect of compaction is limited. In the second stage, thedeformation modulus is decreasing slowly during a longperiod, which occupies most of the cycle time. Di�erentcoring angle specimens show an approximate same trend.�ethird stage corresponds to the accelerated phase of fatiguedeformation, after the sample has experienced several cycles.�e deformation modulus reduced, and the specimendamaged. When the ratio of the upper limit stress is 0.95, thechange in the deformation modulus was very evident.

3.6. Fatigue Fracture Characteristics. Figure 12 shows thetypical failure modes after fatigue test. As seen in Figure 12(a), it indicates that the failure mode of parallel coringsamples was mainly tensile failure mode. �e sampleswere decomposed into smaller parallel blocks, through thetensile failure of multiple layer surfaces, and the micro-cracks had enough time to start, spread, and coalesceunder cyclic loading. �e internal stresses were adjustedand redistributed with the crack initiation and propa-gation, and the whole fracture was divided into a numberof slices. As seen in Figure 12(b), vertical coring specimensformed a number of cut bedding planes after cyclicloading. �e failure mode was mainly mixed failure anddominated by tension cracks and shear cracks. �e mainpart of the specimen fell as a powder on the test platform,with a very full rupture. In the process of hydraulicfracturing, if natural fractures and microfractures arepresent, the fracture pressure required to extend thecracks will be lower; thus, it is easier to form a fracturenetwork.

4. Discussion

Hydraulic fracturing technology is the key technology fordeveloping shale reservoir.

0.3

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Figure 8: �e relationship between axial strain and relative cycle. (a) Parallel coring; (b) vertical coring.

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In hydraulic fracturing engineering, the focus is on thestrength and fracture characteristics of shale, which often referto the value under static loading. However, pressure ¡uctu-ations in hydraulic fracturing ¡uid can be clearly observed,with more than twenty circulates obtained. Hence, the stress-level- and stress-path-dependent deformation and strengthproperty of shale rock, which dramatically di�ers from thatunder monotonic or static load, are of great importance tolong-term stability of the well-bore and form complex frac-ture network. A large number of testing results demonstratethat the peak strength of rocks decreases by 65% due to crackgrowth when cyclic loading is considered [7]. Testing resultsin this paper demonstrate that, under cyclic loading, shalerocks fail at a stress close to 80% of the peak strength undermonotonic loading. Fatigue life increases in a power functionwith decreasing the maximum stress for the two cases. Inaddition to increases in fatigue life, the steady stage slope

decreases. Also the angle between loading direction andbedding plane directly a�ects the characteristics of fatigue test.For parallel coring samples, the end deformation of fatiguetest is consistent with uniaxial loading failure deformation.But the rupture mode is more broken than uniaxial com-pression. �is indicates that, in the process of hydraulicfracturing, the ¡uctuation of water pressure helps to openmore bedding surfaces, to form complex network crack. Forvertical coring samples, the specimen is very fully damagedafter the fatigue load. Analysis of fatigue damage variable atdi�erent load levels shows that when the maximum load levelis decreased, the process of crack initiation occupies a largeproportion of the whole fatigue life, while crack growth occursat higher stresses. When hydraulic fracturing is carried out inthe reservoir of bedding shale reservoir, the original naturallamination seam can be activated by adjusting the pressure ofhydraulic fracturing pump.

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Figure 9: �e relationship between irreversible deformation and the number of cycles. (a) Parallel coring; (b) vertical coring.

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Figure 10: Damage variable versus cyclic numbers. (a) Parallel coring; (b) vertical coring.

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5. Conclusions

In order to understand the fatigue behavior of shale rock,samples were subjected to uniaxial cyclic loading in twoprincipal loading orientations, for di�erent maximum stresslevels and amplitudes. Two failure modes were discoveredthrough the analysis of the fracture morphology, the stress-strain curves under uniaxial compressive and cyclic loading,the e�ect of maximum stress, and the characteristics of

fatigue damage evolution. �e results show that the cyclicloading e�ect is clear. In summary, several conclusions canbe made:

(1) Accumulated fatigue damage occurred in a three-stage process. At di�erent maximum stress level, thefatigue life increases as a power-law function withdecreasing maximum stress, and changing themaximum stress signi�cantly a�ected the fatigue

(a)

(b)

Figure 12: Typical failure modes after fatigue test. (a) Parallel coring; (b) vertical coring.

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10121416182022242628

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10121416182022242628

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us (G

Pa)

(b)

Figure 11: �e relationship between fatigue deformation modulus and relative cycle times. (a) Parallel coring; (b) vertical coring.

8 Advances in Civil Engineering

process; the process of crack initiation occupiesa large proportion of the whole fatigue life, whilecrack growth occurs at higher stresses. At the sametime, different coring angle specimens show differentcharacteristics, for vertical coring samples, the initialdamage is greater under condition of the loadingdirection perpendicular to the bedding surface.

(2) Experimental results demonstrate that, after a fewinitial cycles, the shale rocks exhibited almost elasticbehavior, and the elastic deformation was recoveredin the process of unloading, but the irreversibledeformation remained. With the increasing numberof cycles, the irreversible deformation is observed.*e maximum stress directly affects the axial strainvalue of ultimate failure.

(3) Shale rock is a nonhomogeneous material, and itsfatigue life differs greatly in two principal loadingorientations, which results in great difficulty inconducting fatigue analysis. *e fatigue damageprocess may be divided into 3 stages: an initialdamage stage, a constant velocity damage stage,and an accelerated damage stage which accounted forabout one-third of the total damage. In order to ef-fectively activate the original bedding plane or naturalcracks, first of all, the distribution mode of naturalcracks and bedding plane should be mastered; then,the number of cycle times and the maximum of thepump pressure can be adjusted as far as possible.

Conflicts of Interest

*e authors declare that there are no conflicts of interestregarding the publication of this paper.

Acknowledgments

*e research was supported by the National Natural ScienceFoundation of China (51574218), National Science andTechnology Major Project of China (2016ZX05060-004and 2017ZX05036-003), and Research on Key Scientific andTechnical Issues in Exploration and Development of ShaleGas (XDB10040200), CAS Pilot Project (B). *e authorswould like to express our greatest gratitude for their gen-erous support.

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